1. Field of the Invention
The present invention relates to an image reading apparatus or in particular to the normalization of the read data.
2. Description of Related Art
In a printer or the like wherein an image of full color is reproduced, digital image data R, G, B of three primary colors of red, green and blue which have been read from a document are converted for image reproduction to data C, M, Y of three colors of cyan, magenta and yellow for color reproduction.
The photoelectric conversion data obtained with a linear charge coupled device (CCD) sensor or the like are converted first to digital data with the analog to digital (A/D) conversion, and then the normalizing called the shading correction is performed on the digital data.
In the normalization, the white level is read for example with a standard white plate, while the black level is read for example in a state wherein the image signal is not received. Then, the read data are normalized between the white and black levels for each of the reading elements of the sensor. The shading correction also removes the offset errors.
In the shading correction, if the standard data for the correction is determined from the sampling data of one line and the sampling data include image noises, image noises appear always at the same pixel positions when a document image is read and regular image noises such as a line in the subscan direction are included in a reproduced image. There are many causes of the image noises such as the power supply noises or crosstalk noises of the clock system. Such image noises deteriorate the signal to noise (S/N) ratio of the image signal before the A/D conversion.
In order to remove image noises of the standard data of the normalization, it would be good to average the standard data of a plurality of lines. However, this needs many line memories.
A method for smoothing the standard data in the main scan direction was proposed, but this does not reduce noises sufficiently. Further, there is a disadvantage that ordinary shading distortion cannot be corrected well.